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Note: Large-format versions of the plates for this report are available in a companion document.

One hundred twenty six geophysical logs from locations near and downdip from the current Edwards aquifer freshwater/saline-water interface between San Antonio and Kyle, Texas, were acquired and analyzed. Measured water quality data was obtained from eighty-one water samples taken from wells within the study area. Twenty wells possessed both usable geophysical logs and measured data which were used together to verify a high correlation between estimated and measured water quality parameters. This study and Edwards Underground Water District (EUWD) Reports 92-02 and 92-03 establish good agreement between calculated and measured values of specific conductance and measured and estimated values of total dissolved solids within the study area and near the freshwater/saline-water interface between Uvalde and Kyle, Texas.

The location of the freshwater/saline-water interface derived from this study closely parallels data published by the EUWD, such as EUWD Bulletin 51 (1992),  but indicates that there is slightly less area occupied by freshwater than previously shown. Maps presented in this report can be used to infer additional water quality characteristics near the interface such as rate of change of total dissolved solids, concentrations of highly mineralized water, zones of dilution downdip and indications of freshwater flow patterns.

This study suggests that a very large area of moderately saline water exists in eastern Bexar County and western Guadalupe County. The transition from moderately saline water to brine is very rapid near the common corner of Bexar, Atascosa, and Wilson counties. The large area of moderately saline water changes to water containing higher total dissolved solids northward from Guadalupe County toward the San Marcos River. A re-entrant of very saline water, which is subparallel to the San Marcos River, terminates the moderately saline zone in Guadalupe County, extends to the northwest, and is present in the City of San Marcos.